This invention relates to a temperature responsive switch having a stationary contact, a movable contact cooperating with the stationary contact and carried on a bistable (over-the-center) spring which, in turn, is mounted on a setting arm. The switch further includes a bimetal member which, upon heating, can displace the bistable spring so that the movable contact lifts off the stationary contact to place the switch from an ON position to an OFF position. By virtue of the force exerted by the heated bimetal on the bistable spring, the latter may snap over-the-center from a first stable position (in which the bistable spring urges the movable contact into engagement with the stationary contact) into a second stable position (in which the bistable spring urges the movable contact away from the stationary contact). Further, the switch has a setting shaft which, for example, by manual actuation, adjusts the setting arm between an OFF position and a maximum temperature position for determining the temperature value at which the bimetal effects separation of the movable contact from the stationary contact. The temperature responsive switch further includes a plunger which, when held in a spring-stopping position, blocks the motion of the bistable spring at a location between the ON ("contacts closed") position and the dead center for preventing the bistable spring from reaching its dead center and thus snapping from its first stable state into its second stable state.
A switch of the above-outlined known type is a temperature limiting switch used, for example, for controlling cooking plates in cooking ranges or water heaters. A temperature selection is effected by means of an appropriate angular setting of the setting shaft. As a function of such angular setting, the position of the setting arm is varied. Since the setting arm carries the movable contact spring which is designed as an over-the-center bistable spring, a temperature may be set at which the bimetal causes the bistable spring to snap from its first stable state into its second stable state in which the spring maintains the contacts in an open (OFF) position, after having travelled through its dead center. It is not feasible to bring the bistable spring thereafter into a "contacts closed" (ON) position merely by rotating the setting shaft. Thus, by rotating the setting shaft it is not feasible to resume a heating process which was interrupted due to the temperature limiting operation of the switch. Rather, a resumption of the heating process is preconditioned on resetting the bistable spring by means of a separate plunger over its dead center into its original initial position with respect to the setting arm. Such temperature limiting switches have a relatively complex structure, they are not convenient to operate and are furthermore limited in their function.
Further, temperature regulating switches are known which permit heating, for example, of a water heater, to a preselected temperature and then maintain the heated water at that temperature. The structure of such switches is similar to that outlined earlier for temperature limiting switches. In such temperature regulating switches, however, the contact spring is prevented from snapping--urged by the bimetal--over its dead center into the final open position relative to the setting arm. If in such temperature limiting switches the desired temperature is reached, the bimetal moves the contact spring into its OFF position. By virture of the cooling of the bimetal, effected by shutting off the heat, the contact spring, by virtue of its bias, swings back into the ON position whereupon heating proceeds up to the successive switch-off period. The functioning of temperature regulating switches of this type is limited to the heating to a certain temperature and to maintain the temperature at that value.
It is further known to equip a water heater both with a temperature regulating switch and a temperature limiting switch. Such a combination makes possible the following advantageous mode of operation: the temperature regulating switch is effective in a heating range, for example, between 0.degree. and maximum 80.degree. C. In this temperature range the temperature may be maintained on a desired level throughout the entire period in which the heating apparatus is operative, for example, to heat and keep warm nursing bottles or baby food. If the temperature is maintained by a temperature regulating switch above the noted temperature range, there are risks that the water vaporizes. Therefore, in this range, the above-outlined temperature limiting switch is effective which, when the preset temperature level is reached, permanently opens the heating circuit. In everyday use, however, the moment of automatic OFF switching does not always coincide with the time when the heated article, such as water should be available for use at the desired set temperature. If there lapses an appreciable period between the automatic switch-off of the temperature limiting switch and the time when the heated water is to be utilized, the water has to be again heated for a short time to the desired temperature. For this purpose, a resetting plunger has to be depressed. A new energization by depressing the plunger, however, is possible only when the bimetal has already reached a predetermined cooled state. Before such state is reached, the insufficiently cooled bimetal prevents a return of the bistable contact spring over its dead center into its ON position relative to the setting arm.